1. Field
The present invention is directed generally to thermodynamic machines and, more particularly, to machines based on the Stirling thermodynamic cycle.
2. Description of the Related Art
The concept behind conventional Stirling machines such as conventional Stirling engines and Stirling coolers has been known since the 19th century. Early-on, conventional hot air Stirling machines had some commercial success, however, electric motors and internal combustion machines displaced their use by the early 20th century.
The application of modern materials and analysis tools beginning in the mid-20th century allowed for improvements to be made in efficiency, power density and general functionality, which allowed conventional Stirling machines to capture a few specialized niche markets. Also, conventional Stirling machines have the advantages of low noise, high efficiency, ultra-low emissions, and the ability to operate from any high grade heat source. Unfortunately, conventional Stirling machines have drawbacks as well so that they have remained in these niche markets and otherwise a subject of curiosity.
Conventional Stirling machines are produced in three topological configurations generally referred to as alpha, beta, and gamma, and include two basic mechanical implementations: kinematic and free-piston machines. Kinematic machines are characterized by mechanical linkages that impose specific strokes and phase relationships among various power pistons and/or displacer pistons by means of connecting rods, crankshafts, bearings, and sliding seals. Kinematic machines require lubrication of the mechanical linkages and include sliding seals that impact operational lifespan and reliability of the machines due in part to lubricant leaking past the seals and associated heat exchangers becoming fouled.
Conventional free-piston (non-kinematic) machines have existed solely as single cylinder configurations. These single cylinder machines each have one reciprocating power piston and one displacer piston that move independently from one another and are not mechanically coupled to another. The stroke and phase relationships of the power piston and the displacer piston are subject to associated pressure wave interactions and resonant spring/mass/damper characteristics. Versions of conventional free-piston single cylinder machines can be built to forego requirements for lubricants and rubbing seals, which allows for very long term operation with high reliability. The single cylinder free-piston machines are also in general mechanically simpler than kinematic machines, but design for proper operation requires very sophisticated dynamic analysis and fine tuning adjustments to enable the single reciprocating piston and the single displacer to operate in the proper phase relationship at the full desired stroke without overstroke.
Single cylinder free-piston Stirling machines are elegantly simple mechanically, but extremely complex from a dynamic and thermodynamic analysis perspective. This complexity is evidenced by conventional efforts over the last approximately forty years. During those years roughly dozens of organizations have attempted to produce single cylinder free-piston Stirling machines and only a few are known to have had any significant success.
Possibly less than a handful of profitable practitioners of single cylinder free-piston Stirling machines worldwide remain. Conventional single cylinder free-piston Stirling machines are single-acting beta or gamma configurations with a displacer piston and a power piston. The displacer is a typically lightweight, lightly damped driven resonant harmonic oscillator. The power piston is a typically massive, heavily damped (as a result of extracting useful work) driven resonant harmonic oscillator.
The only coupling between displacer and power piston is the dynamic pressure wave generated by the displacer shuttling working fluid between the hot and cold regions of the Stirling machine. The two resonant harmonic oscillators must be properly “tuned” by carefully selecting parameters that affect moving masses, spring rates, and displacer drive rod area, and by displacer damping as a result of fluid flow losses through heater, regenerator, cooler, and connecting passages.
The desired outcome of the above tuning is to ensure that both the displacer piston and the power piston operate at full stroke but avoid overstroke at all times, including any potential transients, and that the phase lag between them produces near-optimum power transfer. The net result is that single cylinder free-piston Stirling machines are extremely sensitive to having all these tuning parameters in proper balance to produce and maintain proper operation. Many seemingly minor deviations can produce major performance, or even functional, degradation. The free-piston single cylinder Stirling machines are also unfavorably heavy for a given rated capacity and have limitations in their peak output capacity.
In general, kinematic Stirling machines less than about a kilowatt in output are single cylinder machines of the beta or gamma configuration with one piston and one displacer, which may be in the same or different cylinders (the alpha configuration requires by definition at least two piston cylinders with one piston per cylinder). Larger machines may be a kinematic two-cylinder (two-piston) alpha configuration in the lower power levels, but nearly all with 10-kW or higher output are kinematic four-cylinder alpha machines with four sets of heat exchangers (heater/regenerator/cooler) interconnecting the four pistons in the so-called Siemens or Rinnia configuration as illustrated schematically in FIG. 1 showing a conventional kinematic multicylinder implementation. The implementation 10 includes non-fluid non-free-pistons 12 with piston rods 14 coupled to a crankshaft 16.
In general conventional single-cylinder and two-cylinder machines are single-acting (Stirling-cycle pressure wave on one end of the piston and near-constant pressure on the other end of the piston) machines while in general four-cylinder kinematic machines are double-acting (different Stirling-cycle pressure waves on each end of the piston with a 90 degree phase lag between the pressure waves imposed by the kinematic mechanism). These factors cause the kinematic alpha machines, particularly in the double-acting configuration, to have a significantly higher power density and typically higher efficiency than the beta or gamma configurations. Although benefits exist, problems as described with Stirling machines continue to interfere with further commercialization potential of the technology.